Device and method for margin testing a semiconductor memory by applying a stressing voltage simultaneously to complementary and true digit lines

ABSTRACT

A margin test on a Dynamic Random Access Memory (DRAM) in accordance with the invention begins with a supply voltage level being stored in all memory cells of the DRAM. Circuitry incorporated into each sense amplifier of the DRAM then isolates the digit line equilibrating circuitry in each sense amplifier from the cell plate voltage DVC 2  or supply voltage V CC  to which the equilibrating circuitry is normally connected and connects the equilibrating circuitry to ground instead. The equilibrating circuitry is then activated for a predetermined refresh interval of about 150 to 200 milliseconds to equilibrate the true and complementary digit lines in each digit line pair of the DRAM to ground for the refresh interval. This stresses all the memory cells in the DRAM with a V CC -to-ground voltage drop for the entire refresh interval. The DRAM is then restored to normal operations and all the memory cells in the DRAM are read to identify any that leaked too much charge during the refresh interval, which identifies any memory cells that failed the margin test and require repair.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/392,154,filed Sep. 8, 1999 now U.S. Pat. No. 6,101,139, issued Aug. 8, 2000,which is continuation of application Ser. No. 09/026,244, filed Feb. 19,1998, now U.S. Pat. No. 6,002,622, issued Dec. 14, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to semiconductor memories and, morespecifically, to devices and methods for margin testing a semiconductormemory by, for example, equilibrating all complementary and true digitlines of the memory to ground simultaneously during a test mode.

2. State of the Art

As shown in FIG. 1, one conventional method for margin testing asub-array 10 of a semiconductor memory begins with storing a supplyvoltage V_(CC) on all memory cell capacitors 12 of the sub-array 10.This is accomplished by writing logical “1” bits to memory cellsattached to true digit lines D0, D1, etc. using sense amplifiers 14 andeven row lines R0, R2, etc., and by writing logical “0” bits to memorycells attached to complementary digit lines D0*, D1*, etc. using thesense amplifiers 14 and odd row lines R1, R3, etc. A cell plate voltageDVC2 equal to one-half the supply voltage V_(CC) is applied to the cellplate of each memory cell capacitor 12.

Once each memory cell capacitor 12 has stored the supply voltage V_(CC),the row line R0, for example, is fired, causing the memory cellsattached to the row line R0 to dump their stored charge from theirmemory cell capacitors 12 onto the true digit lines D0, D1, etc. Thiscauses the sense amplifiers 14 to pull each of the true digit lines D0,D1, etc. up to the supply voltage V_(CC), and to pull each of thecomplementary digit lines D0*, D1*, etc. down to ground. As a result, afill V_(CC)-to-ground voltage drop is imposed across NMOS access devices16 of the memory cells attached to the complementary digit lines D0*,D1*, etc. The V_(CC)-to-ground voltage drop is maintained across theNMOS access devices 16 for a predetermined refresh interval of typicallyabout 150 to 200 milliseconds (ms). This stresses any “leaky” NMOSaccess devices 16 and causes any such NMOS access devices 16 to losesignificant charge from their memory cell capacitors 12 to thecomplementary digit lines D0*, D1*, etc. to which they are attached.

Once the predetermined refresh interval is over, all of the memory cellsattached to the complementary digit lines D0*, D1*, etc. are read. Anyof these cells that read out a logical “1” bit as a result of leakingexcessive charge, instead of reading out the logical “0” bit theyoriginally stored, are flagged as failing the margin test.

Once the memory cells attached to the complementary digit lines D0*,D1*, etc. have been tested, the memory cells attached to the true digitlines D0, D1, etc. are tested by firing the row line R1, for example.This causes the memory cells attached to the row line R1 to dump theirstored charge from their memory cell capacitors 12 onto thecomplementary digit lines D0*, D1*, etc. In turn, this causes the senseamplifiers 14 to pull each of the complementary digit lines D0*, D1*,etc. up to the supply voltage V_(CC), and to pull each of the true digitlines D0, D1, etc. down to ground. As a result, a full V_(CC)-to-groundvoltage drop is imposed across NMOS access devices 18 of the memorycells attached to the true digit lines D0, D1, etc. The V_(CC)-to-groundvoltage drop is maintained across the NMOS access devices 18 for anotherpredetermined refresh interval of about 150 to 200 ms. This stresses any“leaky” NMOS access devices 18 and causes any such NMOS access devices18 to lose significant charge from their memory cell capacitors 12 tothe true digit lines D0, D1, etc. to which they are attached.

Once the predetermined refresh interval is over, all of the memory cellsattached to the true digit lines D0, D1, etc. are read. Any of thesecells that read out a logical “0” bit as a result of leaking excessivecharge, instead of reading out the logical “1” bit they originallystored, are flagged as failing the margin test.

This conventional margin testing method thus typically takes twopredetermined refresh intervals of about 150 to 200 ms. each tocomplete. Since row lines in different sub-arrays in a semiconductormemory typically cannot be fired simultaneously because the addressingof the row lines is local to their respective sub-arrays, thisconventional method cannot be used on more than one sub-array at a time.As a result, in a semiconductor memory containing four sub-arrays, forexample, the conventional method described above takes approximately 1.2to 1.6 seconds to complete. Because of the large number of semiconductormemories that typically require margin testing during production, itwould be desirable to find a margin testing method that can be completedmore quickly than the method described above.

As shown in FIG. 2, another conventional method for margin testing asemiconductor memory has been devised to conduct margin tests morequickly than the method described above. In this method, a senseamplifier 20 includes equilibrating NMOS transistors 22 whichequilibrate true and complementary digit line pairs D0, D0*, etc. to abias voltage V_(BIAS) on an equilibrate bias node 23 in response to anequilibrate signal EQ. It should be understood that an “equilibrate biasnode” is a node to which a cell plate is coupled, and from which a biasvoltage is globally distributed for use by equilibrating transistorsthroughout a semiconductor memory.

When the semiconductor memory is not in a margin test mode, a test modesignal TESTMODE is inactive, so that a PMOS transistor 24 is on and thebias voltage V_(BIAS) on the equilibrate bias node 23 is equal to thecell plate voltage DVC2 on the cell plate 25. When the semiconductormemory is in a margin test mode, the test mode signal TESTMODE is activeso that the PMOS transistor 24 isolates the equilibrate bias node 23from the cell plate 25, and so an NMOS transistor 26 connects theequilibrate bias node 23 to a probe pad 28 positioned on the exterior ofthe semiconductor memory. A stressing voltage, such as ground, can thenbe applied to the probe pad 28 during margin testing to simultaneouslystress memory cells attached to both the true and complementary digitlines D0, D0*, etc.

Because the method described immediately above does not require thefiring of any row lines in order to stress memory cells, all cells in asemiconductor memory can be stressed at once using this method. As aresult, this method only requires one predetermined refresh period tocomplete testing, no matter how many sub-arrays a semiconductor memorycontains. Thus, the method dramatically improves the speed with whichmargin testing can be completed.

Unfortunately, the method described above with respect to FIG. 2 hasproven difficult to implement because the PMOS transistor 24 generallydoes not reliably isolate the equilibrate bias node 23 from the cellplate 25. In addition, the probe pad 28 has proven to be a cumbersomeaddition to the exterior of a semiconductor memory.

Therefore, there is a need in the art for a device and method for margintesting a semiconductor memory that avoid the problems associated withthe probe pad method described above while still providing a rapidmargin testing device and method.

SUMMARY OF THE INVENTION

Circuitry, in accordance with the invention for margin testing asemiconductor memory, such as a Dynamic Random Access Memory (DRAM),includes switching circuits formed within the memory. Each switchingcircuit can be conveniently incorporated into a sense amplifier of thememory, and each is associated with a pair of digit lines of the memoryto which it selectively applies a stressing voltage at substantially thesame time during a margin test mode of the memory. The stressing voltagecan be ground when all the memory cells of the memory store a supplyvoltage level during the margin test, or it can be at the supply voltagelevel when all the memory cells store a ground voltage level during themargin test. The switching circuits can apply the stressing voltage tothe digit line pairs through equilibrating circuitry in the senseamplifiers into which they can be incorporated, or can apply thestressing voltage through other means. Also, isolating circuitry can beprovided to isolate the digit line pairs during the margin test from anequilibrate bias node of the memory from which they normally receive adigit line equilibrating bias voltage, such as a cell plate voltage orthe supply voltage.

The inventive margin testing circuitry thus stresses all memory cells inthe memory at the same time without the need to fire individual rowlines in different sub-arrays. As a result, it substantially reduces thetime it takes to complete a margin test of the memory. Also, bypositioning the switching circuits within the memory, the inventivemargin testing circuitry avoids the cumbersome nature of the externalprobe pad of prior margin testing devices.

In other embodiments of the invention, a semiconductor memory, anelectronic system, and a semiconductor substrate (e.g., a semiconductorwafer) incorporate the inventive margin testing circuitry describedabove.

In another embodiment of the invention—a method of margin testing aDRAM—a high voltage level is stored in memory cells of the DRAM.Equilibrating circuitry in sense amplifiers of the DRAM is isolated froman equilibrate bias node of the DRAM and from a cell plate voltagethereon, and a ground voltage from within the DRAM is applied to theequilibrating circuitry in each sense amplifier. Digit line pairs of theDRAM are then equilibrated to the ground voltage using the equilibratingcircuitry in each sense amplifier, and the digit line pairs are held atthe ground voltage for a predetermined refresh interval in order tostress the memory cells of the DRAM, which are attached to the digitline pairs. After the predetermined refresh interval has passed, all thememory cells of the DRAM are read to identify those that have failed themargin test. The refresh interval may be, for example, about 150 to 200milliseconds.

In still another embodiment of the invention—a method of testing asemiconductor memory—a substantially identical logic voltage is storedin all memory cells of the semiconductor memory. Also, digit line pairsof the semiconductor memory that are attached to the memory cells areisolated from a digit line equilibrating bias voltage. This isaccomplished by deactivating an NMOS transistor coupled between the biasvoltage and the digit line pairs, or by failing to activate equilibratecircuitry coupled between the bias voltage and the digit line pairs thatnormally is activated during memory operations of the semiconductormemory. A stressing voltage from within the semiconductor memory that issubstantially different than the logic voltage stored in the memorycells is then applied to all the digit lines of all the digit line pairsat substantially the same time, thereby stressing the memory cells. Thedigit line pairs are held at the stressing voltage for a predeterminedinterval, and all the memory cells of the semiconductor memory are thenread to identify those that have failed the test.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic and block diagram illustrating a conventionalmethod for margin testing a semiconductor memory;

FIG. 2 is a schematic diagram illustrating another prior art method formargin testing a semiconductor memory;

FIG. 3 is a schematic and block diagram illustrating circuitry formargin testing a Dynamic Random Access Memory (DRAM) according to thepresent invention;

FIG. 4 is a more detailed schematic view of the circuitry of FIG. 3;

FIG. 5 is a block diagram of an electronic system including a memorydevice that incorporates the DRAM and circuitry of FIG. 3; and

FIG. 6 is a diagram of a semiconductor wafer that incorporates the DRAMand circuitry of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 3, the invention includes margin testing circuitry (seeFIG. 4) incorporated into sense amplifiers 30 of a Dynamic Random AccessMemory (DRAM) 32 or other semiconductor memory. It should be understoodby those having skill in the technical field of the invention that themargin testing circuitry may be directly connected to digit lines D0,D0*, D1, D1*, etc. of the DRAM 32 instead of being incorporated into thesense amplifiers 30, or may be incorporated into other circuitry of theDRAM 32 that is connected to the digit lines D0, D0*, D1, D1*, etc.

A margin test is performed on the DRAM 32 in accordance with theinvention by first storing a supply voltage V_(CC) level in the storagecapacitors 34 of the DRAM 32 using the sense amplifiers 30, digit linesD0, D0*, D1, D1*, etc., row lines R0, R1, R2, R3, etc., and NMOS accessdevices 36 of memory cells of the DRAM 32. An active margin test modesignal GNDDIGTM* then causes the sense amplifiers 30 to ground the digitlines D0, D0*, D1, D1*, etc. for a predetermined refresh interval ofabout 150 to 200 milliseconds. Of course, longer or shorter refreshintervals may also be used. Grounding the digit lines D0, D0*, D1, D1*,etc. stresses the NMOS access devices 36 with a V_(CC)-to-ground voltagedrop, causing any of the NMOS access devices 36 that are leaky to leakcharge.

After the predetermined refresh interval has passed, all the memorycells of the DRAM 32 are read. Any that leaked sufficient charge to readout at a low voltage level rather than at the supply voltage V_(CC)level they originally stored are then identified as having failed themargin test.

It should be understood that the invention grounds all digit lines D0,D0*, D1, D1*, etc. at the same time without the need to fire any of therow lines R0, R1, R2, R3, etc. As a result, the invention margin testsall sub-arrays within the DRAM 32 at the same time, in contrast to thetraditional margin test method previously described.

It should also be understood that stressing voltages other than groundmay be applied to the digit lines D0, D0*, D1, D1*, etc. during a margintest, and that, accordingly, different voltages may be stored on thestorage capacitors 34 to begin the margin test. For example, the storagecapacitors 34 may store a ground voltage level while a supply voltageV_(CC) level is uniformly applied to the digit lines D0, D0*, D1, D1*,etc. to stress the NMOS access devices 36.

As shown in FIG. 4, one of the sense amplifiers 30 includesequilibrating NMOS transistors 40 for equilibrating the voltage on thedigit lines D0, D0*. During normal memory operations, the margin testmode signal GNDDIGTM* is inactive, which causes an isolating NMOStransistor 42 to be active and couple the equilibrating NMOS transistors40 to an equilibrate bias node 44 connected to the cell plate voltageDVC2. During the margin test mode, the margin test mode signal GNDDIGTM*is active, causing the isolating NMOS transistor 42 to isolate theequilibrating NMOS transistors 40 from the equilibrate bias node 44, andcausing a PMOS switching transistor 46 to turn on and activate an NMOSswitching transistor 48, thereby applying a ground voltage to theequilibrating NMOS transistors 40. In response to an active equilibratesignal EQ, the equilibrating NMOS transistors 40 in turn apply theground voltage to the digit lines D0 and D0* simultaneously.

It should be noted that because the invention does not attempt toisolate the equilibrate bias node 44 from the cell plate voltage DVC2,but rather provides an isolating NMOS transistor 42 for each senseamplifier 30 to individually isolate each pair of digit lines from theequilibrate bias node 44, the invention provides a more reliable margintest method than the known probe pad method previously described, whichdoes attempt to isolate the equilibrate bias node 44 from the cell platevoltage DVC2 and the cell plate.

In an alternative system as described above, in which the switchingtransistors 46 and 48 are directly connected to digit lines D0 and D0*rather than being connected through equilibrating NMOS transistors 40,or are incorporated into circuitry other than the sense amplifier 30that is directly connected to the digit lines D0 and D0*, an alternativemethod for isolating the digit lines D0 and D0* from the node 44 inaccordance with the invention involves not activating the equilibratesignal EQ.

As shown in FIG. 5, an electronic system 50 in accordance with theinvention includes an input device 52, an output device 54, a processordevice 56, and a memory device 58 incorporating the DRAM 32 of FIG. 3.As shown in FIG. 6, the DRAM 32 of FIG. 3 is fabricated on asemiconductor wafer 60. Of course, it should be understood thatsemiconductor substrates other than a semiconductor wafer also fallwithin the scope of the present invention, including, for example,Silicon-on-Sapphire (SOS) substrates, Silicon-on-Glass (SOG) substrates,and Silicon-on-Insulator (SOI) substrates.

Although the present invention has been described with reference toparticular embodiments, the invention is not limited to these describedembodiments. Rather, the invention is limited only by the appendedclaims, which include within their scope all equivalent devices ormethods that operate according to the principles of the invention asdescribed.

What is claimed is:
 1. A Dynamic Random Access Memory (DRAM) devicecomprising: at least one DRAM memory array including a plurality ofmemory cells and a plurality of pairs of true and complementary digitlines coupled to the plurality of memory cells; and a plurality of senseamplifiers, each sense amplifier of said plurality of sense amplifiersincluding: isolating circuitry coupled between an equilibrate bias nodeof the DRAM device and one pair of digit lines of the plurality of pairsof digit lines of the at least one DRAM memory array isolating the onepair of digit lines from the equilibrate bias node and a cell platevoltage thereon during a margin test mode of the DRAM device; andswitching circuitry coupled to the one pair of digit lines providing aground voltage thereto during the margin test mode stressing all theplurality of memory cells of the at least one DRAM memory arraysubstantially simultaneously.
 2. The DRAM device of claim 1, whereineach of said switching circuitry comprises one or more NMOS transistors.3. The DRAM device of claim 1, wherein the cell plate voltage comprisesone-half a supply voltage of the DRAM device.